Crushed between two diamonds, a compound of hydrogen, sulfur, and carbon superconducts at room temperature.
ADAM FENSTER
Fulfilling a decades-old quest, this week researchers report creating the first superconductor that does not have to be cooled for its electrical resistance to vanish. There’s a catch: The new room temperature superconductor only works at a pressure equivalent to about three-quarters of that at the center of Earth. But if researchers can stabilize the material at ambient pressure, dreamed-of applications of superconductivity could be within reach, such as low-loss power lines and ultrapowerful superconducting magnets that don’t need refrigeration, for MRI machines and maglev trains.
“This is a landmark,” says Chris Pickard, a physicist at the University of Cambridge. But the extreme conditions of the experiment mean that even though it was “pretty spectacular,” says Brian Maple, a physicist at the University of California, San Diego, “this is certainly not going to be useful in making a device.”
The announcement, by a team led by physicist Ranga Dias of the University of Rochester, culminates a long march up the thermometer. Superconductivity was first discovered in 1911 by Dutch physicist Heike Kamerlingh Onnes in a mercury wire chilled to 4.2° above absolute zero, or 4.2 K. In 1957, physicists John Bardeen, Leon Cooper, and Robert Schrieffer explained the phenomenon: Their “BCS theory” suggested an electron zipping through a superconductor temporarily deforms the material’s structure, pulling another electron behind in its wake without resistance.
In 1986, a pair of physicists found that in different materials, copper oxide ceramics, superconductivity set in at a higher “critical temperature,” or Tc, of 30 K. Other groups quickly cooked up related ceramic recipes; by 1994, they pushed the Tc up to 164 K in a mercury-based copper oxide under pressure. Electrons also pair up in cup-rate superconductors, but just how they superconduct remains murky.
In 1968, Neil Ashcroft, a theorist at Cornell University, had suggested a different type of material should display BCS superconductivity above room temperature: hydrogen under intense pressure. Numerous groups have claimed to make such metallic hydrogen, using diamond anvil cells, palm-size devices in which a target substance gets crushed to enormous pressure between the tips of two diamonds. But the results remain controversial, in part because the pressures—exceeding those at the center of Earth—are so high they typically crack the diamonds. In 2004, Ashcroft suggested binding hydrogen to another element might add a sort of “chemical precompression” that could make higher temperature superconductivity possible at lower pressures.
The strategy worked. In 2015, researchers led by Mikhail Eremets at the Max Planck Institute for Chemistry reported in Nature that they discovered superconductivity at 203 K in H3S compressed to 155 gigapascals (GPa), more than 1 million times Earth’s atmospheric pressure. Over the next 3 years, Eremets and others boosted the Tc as high as 250 K in hydrogen-rich compounds containing lanthanum. But release the pressure, and all those compounds disintegrate.
Dias and his colleagues thought they could push the Tc up even higher by adding a third element: carbon, which forms strong bonds with neighboring atoms. “We were flying blind,” says team member Ashkan Salamat, a physicist at the University of Nevada, Las Vegas.
They loaded their diamond anvil cell with tiny solid particles of carbon and sulfur milled together, and then piped in three gases: hydrogen, hydrogen sulfide, and methane. They then shined a green laser through the diamond, triggering a chemical reaction that turned the mixture into transparent crystals.
When they then cranked up the pressure to 148 GPa and checked the conductivity of the sample via electrical leads, they found that the crystals became superconducting at 147 K. By increasing the pressure to 267 GPa, the team reached a Tc of 287 K, the temperature of a chilly room or an ideal wine cellar. Magnetic field measurements also indicated the sample had become superconductive, Dias and his colleagues report this week in Nature.
“The results look believable,” Eremets says. He notes, however, that the Rochester team has not yet been able to determine the precise structure of the compound that’s superconducting. Researchers will soon set to work on that question, and they will likely also start to substitute other elements into three-component hydrogen-based mixtures in hopes of even higher temperature superconductors. “That’s the next thing everyone is going to be doing,” says Eva Zurek, a theorist at the University at Buffalo.
The ultimate aim, Eremets adds, is to find a room temperature superconductor that is stable when the pressure is released. If researchers pull that off, the results could transform daily life. Dias says: “I think this is actually possible.” But theory doesn’t yet suggest a way to make hydrogen-based materials work at ambient pressures. Zurek adds, “We don’t necessarily have a clear path forward.”